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Ultraviolet images from Cassini reveal detail of Saturn's rings

By Robert Roy Britt, SPACE.com

Saturn's rings are a lot dirtier than originally believed, according to new observations by the Cassini spacecraft that show that the inner regions are packed with rock and mud.

The "Cassini division" in faint red at left is followed by the A ring in its entirety.

University of Colorado, LASP/AP

"We've known for decades the rings are mostly made of water," said Larry Esposito of the University of Colorado at Boulder. "Now we know the amount of water varies, increasing toward the outer edge of the rings.

Cassini images made in ultraviolet light and released yesterday have been combined with infrared pictures — all taken during the June 30 pass through the ring plane — to give scientists a fresh but still puzzling picture of the majestic structures.

Gunk on ice

Scientists don't know how the rings formed. One idea is that an icy object from the outer solar system was lured in close and broken apart by Saturn's impressive gravity. The new data are not conclusive, but they can be read to support that theory.

The ice in the rings is "like gunk in a skating rink," Esposito told SPACE.com. He described it also as being like mud.

Nobody knows what the gunk is made of, he explained, but it is likely silicates and organic material, the stuff of rocks and dirt on Earth. The ice is also thought to contain water mixed with other frozen substances such as ammonia.

Cassini's cameras were not powerful enough to resolve the individual icy boulders and smaller particles thought to make up the rings. The new ultraviolet (UV) observations were compared with similar observations made in laboratories, where researchers determined how much UV light would be reflected by water versus other material.

Esposito stressed that the new observations are not surprising. But he said they are the most detailed ever made in UV and infrared, providing data that will be chewed on for some time.

Meanwhile, the differing concentrations of dirt and water might tell something of the rings' origins and evolution.

Bombardment

Esposito thinks the rings formed long ago as icy material that has since been bombarded by dark stuff from meteors. The darker material might be more diluted where ice concentrations are higher, he said.

Saturn's moons might also play a role. They perturb particles in the rings by creating "density waves" of gravity spotted by Voyager 2 in 1981 and in other new observations by Cassini. A moon outside a ring pulls stuff outward as it orbits, for example, causing particles to clump together. The density waves could be locations where more water ice resides, Esposito said, but its not clear yet if that's the case.

Esposito said the gunk in the rings has a UV signature similar to dark material on Saturn's moon Phoebe, observed last month.

"Whatever has colored Phoebe may also have been a part of the rings or may have fallen on the rings," he said. "But of course we don't know what Phoebe is made of."

The Cassini spacecraft was named for Domenico Cassini, who in the 17th Century discovered the Cassini division in Saturn's rings. The rings were originally spotted by Galileo shortly after the invention of the telescope.

The ring system begins from the inside out in this order: D, C, B, A, F, G, E. Esposito discovered the F ring in 1979 using Pioneer 11 data.